Well THAT didn't last long...
I just deleted the virtual machine on which I'd installed the PDC build of Longhorn (last night), and uninstalled the Virtual PC 2004 beta. I'm not sure whether it was simply the fact of running such an early alpha on a beta VM, or if the fact that I was trying to do it on a laptop with a 4200 RPM hard drive and a slow bus (though a 2ghz P4 proc) had more to do with it, but despite having read reports of slow performance, I found running the alpha on Virtual PC to be completely unusable.
Now, I'm used to running with VMs, having used VMWare for every piece of alpha and beta software I've tested for the last 2-3 years, and I'm used to the performance sacrifices associated with this choice. But with this particular combo (and, again, I'm willing to entertain the notion that my hardware may be a contributor), the performance is excruciatingly slow. So much so that I can't imagine how long it would take to install the VS Whidbey preview and Longhorn SDK. So I'm shelving my Longhorn exploration until I can buy or put together a machine dedicated to that purpose.
Just wanted to give folks a heads-up in case you're getting ready to run the Longhorn preview. Be prepared for poor performance if you're running it in a VM, even if you've got lots of RAM (I've got 1Gb on my laptop, and devoted 512MB to the VM). And if you're running on a slow disk, it will probably be even worse. Also, I noted that each time I ran VPC, I ended up having to reboot my host PC when finished with the VM to get rid of all of the performance hiccups, even after shutting down VPC. Not surprising, given running an alpha OS on a beta VM system, but still kinda annoying.
Not trying to be overly negative here, just wanting to share my recognition that trying to do too much with too few resources isn't always the most efficient course.
UPDATE: Julie writes that she wants to “play with longhorn without installing it”. I can't say as I blame her, but I want to be clear that I'm not recommending that people avoid installing the Longhorn PDC bits, just that they have realistic expectations about performance, particularly when running on a VM. Just because I don't currently have the hardware resources available to make that acceptable doesn't mean it's not possible, and I sure wouldn't want to spoil anyone's fun playing with the cool new bits. That said, Julie may be onto something.